Oligocene Laccoliths and Sedimentary Rock Domes of the Henry Mountains

During exploration of the Henry Mountains in 1875, G.K. Gilbert (1843-1918) postulated a process through which magma rose through earth’s lithosphere in a dike, spread laterally as a sill, and opened a thick chamber by lifting and bending the overlying strata. He gave these chambers of ‘molten rock’ the name “laccolite” (now, laccolith) and described associated dikes, uplifted sills, and domed sedimentary strata (Gilbert, 1877; Hunt, 1988). Geological maps of the southern Henry Mountains (Mount Holmes, Mount Ellsworth, and Mount Hillers) show the laccolithic intrusions are circular in plan and formed at similar depths, 3-4 km. The overlying sedimentary rock domes have similar radii, 5–7 km, but their amplitude increases from 1.2 to 1.8 to 2.5 km, respectively. Limb dips steepen from 20° to 50°–55° to 75°–85°, and thus record serial stages in the deformation of strata over the growing laccolithic intrusions (Jackson and Pollard, 1988). The Henry Mountains are the original and now classic examples of magmatic structures building mountains within a broader typology of mountain belts worldwide. Gilbert’s investigation exemplifies a scientific methodology that progresses from field observations to idealizations to physical principles to mechanical models to solutions tested using field data (Pollard and Martel, 2020).